Wind turbine generators can be very large and accordingly very heavy. For example a direct-drive generator for a horizontal axis 3.0 megawatt rated wind turbine can weigh in the order of 50-80 tons or more. The assembly of such a generator can involve first assembling a rotor structure with magnets, and assembling a stator structure with windings. These must then be merged or “married”, and a bearing is mounted so that, for example in the case of an outer rotor machine, the rotor can move freely about the stator. Such a merging is usually carried out by supporting the rotor housing such that its axis of rotation is horizontal, and then inserting the horizontally-held stator into the rotor interior cavity. Since the rotational axis is horizontal throughout the merging, this procedure is generally referred to as “horizontal marriage”. Various critical steps must be carried out during the marriage to ensure that the rotor, stator and bearing are all exactly positioned relative to each other and that the overall assembly is secure. A magnet loading step must also be carried out, for example to load permanent magnets onto the field arrangement of the generator, usually the rotor. When permanent magnets are used, extreme forces act on various components of the generator, and on other already loaded magnets, owing to the powerful magnetic fields. The magnetic forces and also the weight of the rotor housing can act to distort the rotor housing during the magnet loading procedure. However, it is of paramount importance that a gap of usually only a few millimeters is maintained between the field windings and the magnets during assembly. Known assembly techniques involve the use of spacers, hydraulic or other position and orientation adjustment actuators and sensors to avoid any departure from a predetermined minimum/maximum distance between rotor and stator during marriage. Such techniques might also involve the use of mechanical guidance means such as rails to guide the passage of the rotor and/or stator.
A further challenge associated with large structures such as a cylindrical hollow rotor of several meters in diameter is the inherent lack of stiffness and resulting deformation. The weight of the rotor itself can cause considerable deformation. To avoid such deformation during the marriage procedure, a support structure such as a support ring can be temporarily mounted about the rotor structure to maintain its circular perimeter. Such support rings require maintenance, servicing, and storage, and therefore add to the overall cost of construction. Furthermore, such parts can fail and result in an accidental contact between rotor and stator during marriage. A deformation of the rotor can result in a misalignment between connecting parts such as rotor, bearing and stator due to an erroneous offset at their interfaces. A further problem lies in mounting bolts through matching holes in stator, rotor and bearing in order to interconnect these, since the matching holes, which of necessity may have very little clearance, must be aligned with a very high degree of precision in order for a bolt to pass through. A further problem in such a vertical marriage is the centring of such heavy components. A further challenge in vertical marriage is that, in the merging of stator and rotor, large and very rigid handling and support structures are required for each of the rotor and stator. These support structures, although cumbersome and heavy, must be very precisely adjustable in the millimeter range, and these present a further significant cost factor. For these reasons, the known methods of assembling large generators are very time-consuming and cost-intensive.